As a former leader of the youth movement that overthrew Slobodan Miloševic—and now as somebody who shares his passion about non-violent struggle with everyone from street activists in the Middle East to students at Harvard and NYU—there’s one thing I’ve learned: A big part of a movement’s success will be determined by the battles it chooses to fight, and a lot of that has to do with how well it understands its opponent. Many centuries ago, Sun Tzu reflected on this idea when he told readers of The Art of War how important it is to always put your strong points against your enemy’s weak points. Take Gandhi, who went up against the British army, the most powerful in the world, by attracting 10,000 Indians to march for tax-free salt—a mineral essential for human survival and found in almost every household, no matter how poor.

That’s the reason you see so many activists campaigning for better and healthier food: Because no matter what a person’s religion, skin color or political belief may be, there isn’t a single human being out there who doesn’t need to eat. Whether it’s food or some other basic necessity, activists who can identify some everyday thing that speaks to as many people as possible will always have an advantage over those who cling to a much narrower platform.

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Which brings us to Milk. Harvey Milk, that is. Apologies for the pun, but you may have heard about this pioneering politician who was the first openly gay public official in America. If you haven’t, he is wonderfully portrayed by Sean Penn in an Academy Award-winning movie called Milk that you may want to check out. Milk’s story is about many things: courage, conviction and dedication. But most of all, it’s about how important it is to start with the small stuff. And by small, I mean dog poop-level small.

Nothing in the first four decades of Harvey Milk’s life suggested that he would one day become an inspiration to anyone seriously interested in human rights and equality. Born on Long Island to a conservative, middle-class Jewish family, he’d known he was gay from a very young age, but took great pains to cover up his true identity. He joined the Navy, fought in Korea and then found work first as an insurance actuary and then as a researcher for a large Wall Street securities firm. This future icon of liberal America even campaigned for the archconservative Republican presidential candidate Barry Goldwater. Milk was hardly a revolutionary, and in fact he once broke off with a boyfriend he dearly loved because he felt the young man was too likely to challenge authority and get in trouble with the police. Milk was successful and respectable, with neatly cropped hair and a closetful of fine suits. He was also miserable, living a lie. Eventually he got fed up: In 1969, at the age of 39, he quit his job, got rid of the tie, let his hair grow and moved west to San Francisco.

The city he found was one busy being reborn. By 1969, it had the largest gay population of any major metropolitan area in the United States. Neighborhoods like the Castro, where Milk eventually settled, were shedding their old residents—working-class Irish Catholics—and welcoming in new ones, young men and women who had come to San Francisco seeking tolerance, free love and flower power. Here Milk felt liberated. Having spent a lifetime keeping his sexuality a secret, he was now accepted openly and wanted to help other gay men and women not to be ashamed of themselves.

Milk, who ran a popular camera shop, soon became involved in local politics. His first stop was the Alice B. Toklas Memorial Democratic Club, the most powerful—and only—gay political organization in town. Milk showed up, smiling widely and talking bravely. He was like so many other young, talented and hugely passionate men and women who decide to make a difference. The way to victory, he and his closest friends believed, was to tell the truth, raise good points, offer sensible solutions and count on good people to come out and vote for change.

But it wasn’t so simple. Back then, even in San Francisco, homosexuality was still a taboo subject. Today, with the advance of gay marriage and the growing acceptance of homosexuality in American society, it’s easy to forget how different the cultural landscape was when Harvey Milk ran for office. In the early 1970s, when Milk was first mobilizing, gay sex was still a felony in many places and a legitimate cause for eviction from rented apartments. As late as 1973, the American Psychiatric Association categorized homosexuality as a mental disorder. Being gay wasn’t something that people were comfortable with. So Milk was running a principled platform that confused, turned off and even revolted plenty of ordinary voters.

His campaign was, of course, a disaster. Milk had no money, no staff and no idea how to run an effective campaign. He did manage to get the support of some gay business owners tired of police harassment, and his personal charm helped win over a handful of converts. But when he finally ran for city supervisor in 1973, he came in 10th out of 32 candidates. But Milk persevered. He discovered a talent for rousing speeches and gave them frequently, talking about persecution and the injustices of anti-gay legislation. He wanted to represent his community, and thought the best way to do that was by organizing all the gays together as one political bloc with a few key allies.

Again he failed. While he had managed to go more mainstream, making inroads with labor unions and firemen and meeting with regular people at bus stops and movie theaters, it still wasn’t enough. This time, although he came closer to victory with a seventh-place showing, a margin of 4,000 voters still guaranteed that Milk would remain little more than a well-meaning and talented niche activist.

Milk needed to attack from a different angle, and even though hard-core evangelical Christians across the country were using San Francisco’s gay community as a stand-in for all that was evil in America, he sought to stand up for his community by focusing on something that all San Franciscans lived in fear of: dog shit.

Matthew Miller's collaborations include Lili Marlene: The Soldiers' Song of World War II (Norton, 2009) and Fortunate Sons: The 120 Chinese Boys Who Came to America, Went to School, and Revolutionized an Ancient Empire (Norton, 2011).